We have characterized the process of spindle pole body (SPB) duplication in wild-type Schizosaccharomyces pombe. (Ding et al., 1997, Mol Biol. Cell). We have also studied various strains of fission yeast that are mutant in genes required for normal mitosis, in order to use 3-D fine structural analysis to characterize mutant phenotype and to understand the functions of the corresponding wild type gene products in the normal cell division process. Cells carrying the mutation, cut11-2 have been reconstructed in 3-D, showing that while the SPB duplicates, one SPB is non-functional. A monopolar spindle is usually formed, but it often fails to attach properly to the nuclear envelope. The cut11+ gene has been cloned, sequenced, and deleted, showing that it is a novel, essential gene encoding a predicted protein with 7 membrane spanning domains. Its product, Cut11p, has been localized to the nuclear envelope in the light microscope, using a fusion protein with the green fluorescent protein (GFP). We have now localized this chimera by immunoEM, using antibodies to GFP and secondary antibodies labeled with 10 nm gold; it is concentrated at the margins of the nuclear pores and of the SPBs during mitosis. We conclude that this protein is an essential part of the mechanism by which objects bind to openings in the nuclear envelope. (In press in Molecular Biology of the Cell) Note that this was the first successful labeling with antibodies to the GFP, yielding a signal-to-noise ratio of about 200. It has served as the model for a series of localization collaborations that are described below. C1